Characterization and evolution of the lowland tropical rain forest of the smallest oceanic Gondwana fragments, with implications for restoration and invasion ecology

Abstract

The Seychelles granitic islands have been small, mid-oceanic, equatorial, mountainous and moist islands for at least 50 million years, with an uninterrupted vegetation cover since their separation from Gondwana. Therefore, unlike hotspot oceanic islands that drastically vary in environmental heterogeneity according to their age, the Seychelles can provide observational data where evolutionary time is de-correlated from habitat heterogeneity. In this paper, we aim to describe for the first time its most widespread, least known, and most threatened ecosystem type: the lowland rain forest. Surprisingly, this had never been done before and the reason is simply that only 6.5 % of those forests have survived untouched by the 2.5 centuries of human presence on these islands. We set six permanent vegetation plots within the largest (ca. 50 ha) and best-preserved relict of this forest, plus four permanent plots in a nearby site (1 km away) that is ecologically homologous but has been intensively modified, abandoned for the last 40 years and which is now a structurally mature late secondary forest. Each plot covered 500 m2 and all vascular plants were inventoried in a series of subplots corresponding to different strata. Within the 0.5 ha of surveyed forest, we observed 35 native species (17 canopy/under-canopy trees, 8 shrubs or small understorey trees, and 10 herbs), of which 11 are endemic (31 %). All plots within the natural forest site were floristically and structurally very similar. Their flora (within just 500 m2) represented about 87 to 92 % of the total flora of the Seychelles lowland mesic forests (defined as an ecological group). In addition, the three most dominant under-canopy trees corresponded to paleo-endemic species having the particularity to be both climax and pioneer trees, which is very unusual. Our study also shows that exotic species were hardly present in undisturbed natural forests but, in disturbed forests, on the other hand, native species were re-colonizing so-called novel ecosystems. Based on these results, we present a list of native species which are appropriate for restoration programs in the Seychelles lowland rain forests, including one that was previously considered as an exotic invasive but which could greatly improve restoration work. Finally, our study sets the basis for long term monitoring of natural ecosystem resilience to invasions on the one hand, and biotic novelty of novel ecosystems on the other.